Since a surface of an electrophotographic photosensitive element (a surface of a photosensitive layer) undergoes various electrical, chemical or mechanical stresses due to processes such as electrification, exposure, development, transference, and cleaning [for example, wear (or abrasion) and scarring of the surface layer due to repetitive use, and oxidization and degradation of the surface due to ozone generated by corona discharge], durability is required for the surface to these stresses. In particular, along with recent popularization of a roller electrification system, it has become a problem that the surface is worn down accompanied with cutting of bonding of molecules on the photosensitive layer surface caused by arc discharge. Further, demands for full-coloration or speedup of a printer and miniaturization of a photosensitive drum bring about overlap of conditions facilitating stresses in the photosensitive element surface as described above. Therefore, improved durability of the electrophotographic photosensitive element has been further required.
In order to solve such problems concerning the photosensitive element surface, improvement in properties such as surface abrasion, a release property of toner, and a cleaning property is attempted by adding a silicone-series compound or fluorine-containing compound which has a small surface free energy and is excellent in water repellency or lubricity [for example, Japanese Patent Application Laid-Open No. 132954/1986 (JP-61-132954A), Japanese Patent Publication No. 113779/1995 (JP-7-113779B)].
However, since these compounds are low in compatibility or dispersibility to a resin constituting a photosensitive layer of the photosensitive element and inferior in transparency of a top surface layer thereof, it is difficult to obtain a high definition image. Moreover, these compounds incline to be maldistribute in the vicinity of the top of the surface layer. Thereby, even if only the top surface layer is slightly worn by friction or sliding in the surface, a property such as lubricity is drastically reduced or a cleaning property is fast deteriorated by bleeding out of these compounds with passage of time. Further, it is difficult to obtain a sharp image over a long period by such deterioration in the lubricating or cleaning property.
Meanwhile, Japanese Patent Application Laid-Open No. 178652/1992 (JP-4-178652A) discloses a method for improving a durability or a repeating property of a photosensitive element, which comprises adding a polysilane or a copolysilane to a photosensitive layer. This document describes that (i) as the polysilane, there may be used a polysilane or copolysilane whose end is blocked with an alkyl group or the like and which has a relatively high molecular weight (in Examples, a number-average molecular weight of 18000, or 23000); (ii) the mixing ratio of the polysilane is preferably about 20% to 80% relative to a binder resin constituting the photosensitive layer [e.g., a poly(methyl methacrylate)]; and (iii) in a single-layered photosensitive element having a combination of a charge transport function and a charge generation function, it is preferred to add 3 to 7 parts by weight of the polysilane and 3 to 7 parts by weight of the binder resin to 1 to 10 parts by weight of a charge-generating substance.
According to the method of this document, however, since the polysilane inferior to the binder resin in mechanical strength is used in great quantities, this method not only is disadvantageous in cost but also accelerates wear (abrasion) of the photosensitive layer. Moreover, the use of the polysilane having a high-molecular weight gives inadequate compatibility or dispersibility to the resin, deteriorates transparency of the photosensitive layer, and has the potential of impairing sharpness (or clearness) in an image.
It is therefore an object of the present invention to provide an electrophotographic photosensitive element improving water repellency and lubricity (lubricating property) thereof and forming a high quality image (or picture image) over a long period, as well as a method for producing the same.
It is another object of the present invention to provide an electrophotographic photosensitive element which has excellent durability without deterioration in a property such as lubricity or a cleaning property even in the case of wearing a surface layer thereof, and a method for producing the same.
It is still another object of the present invention to provide an electrophotographic photosensitive element which can realize a high definition image without deterioration in mechanical strength or transparency and can ensure conservation of a high-quality image property even with prolonged application, a method for producing the same, and an electrophotographic apparatus provided with the electrophotographic photosensitive element.